THE PLAGIOCLASE-BEARING POIKILITIC SHERGOTTITE NORTHWEST AFRICA 12241. A. Udry 1 , A. Ostwald 1 , G. H. Howarth 2 , M. Paquet 3 , L. Forman 4 , J. M. D. Day 3 , and A. Taylor 1 , 1 Department of Geosciences, University of Nevada Las Vegas, 4505 S. Maryland Pkwy, Las Vegas, NV 890154 ([email protected]), 2 Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa, 3 Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093-0244, 4 Space Science & Technology Cen- tre Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia. Introduction: Poikilitic shergottites are gabbro- ic/lherzolitic lithologies and currently consist of 30 paired groups [e.g., 1–5]. Poikilitic shergottites have a bimodal texture that allows us constraints to be placed on magmatic processes from the crust-mantle bounda- ry to the shallow crust [1]. We examined the newly recovered sample Northwest Africa (NWA) 12241, which was classified as an olivine gabbro in the Mete- oritical Bulletin. This meteorite is, in fact, one of the few martian shergottites that contains plagioclase and not solely the high pressure phase maskelynite, and has all of the textural and geochemical characteristics of an ‘intermediate’ poikilitic shergottite. Methods: We conducted quantitative textural anal- yses, bulk major and trace composition analyses, and mineral major element analyses following the same procedure and in the same laboratories as [1]. Electron Backscatter Diffraction (EBSD) analyses were con- ducted at the John de Laeter Centre at Curtin Universi- ty, Western Australia. We collected high resolution maps of plagioclase grains to understand the crystallin- ity of this phase. These data were obtained using step sizes of 0.1–0.5 μm. Results and Discussion: Mineralogy and petro- genesis. Northwest Africa 12241 has a bimodal tex- ture, including: 1) the early-stage poikilitic texture consisting of pyroxene oikocrysts containing olivine and chromite chadacrysts, and 2) a later-stage non- poikilitic texture, containing olivine, pyroxene, plagio- clase, chromite, merrillite, and ilmenite, similar to oth- er poikilitic shergottites [1]. Modal abundances of all phases fall within the range of the other poikilitic sher- gottites [1]. Pyroxene oikocrysts have low-Ca cores with sharp boundaries to high-Ca rims. The low-Ca poikilitic pyroxenes have compositions of En 55–70 Fs 25– 27 Wo 5–18 , whereas high-Ca poikilitic pyroxene rims have compositions of En 44–54 Fs 17–20 Wo 26-39 . Non- poikilitic pyroxenes are distinct grains and are more Ca-rich and more Mg-poor than the pyroxene oiko- crysts, with compositions of En 51 Fs 11 Wo 37 – En 64 Fs 26 Wo 10 . Olivine chadacrysts within pyroxene oikocrysts have a compositions of Fo 64–67 and non- poikilitic olivine have compositions of Fo 60–67 . Lath- shaped plagioclase grains, which have with lengths, up to ~ 750 µm long, are present in NWA 12241 and dis- play undulose extinction and polysynthetic twinnings. Birefringent and amorphous plagioclase have stoichi- ometric composition (5 cations based on 8 O) of An 53– 59 Ab 46–40 Or 2–1 . Some plagioclase grains show a sharp transition from birefringent to isotropic in the same grains, although most plagioclase grains are birefrin- gent. Note that we do not define amorphous plagio- clase as maskelynite as NWA 12241 plagioclase is stoichiometric. Mineral compositions in NWA 12241 and minerals in other intermediate poikilitic sher- gottites are similar in both poikilitic and non-poikilitic textures [1–5]. We conducted olivine-pyroxene-spinel oxybarom- etry and olivine-spinel geothermometry calculations on both NWA 12241 poikilitic and non-poikilitic textures in eight mineral assemblages [6,7,http://melts.ofm- research.org/CORBA_CTserver/Olv_Spn_Opx/index.p hp]. Poikilitic textures have ƒO 2 ranging from -4.3 to - 3.6 relative to the Fayalite-Magnetite-Quartz (FMQ) buffer and non-poikilitic textures have ƒO 2 from FMQ -3.2 to -1.7. The oxygen fugacities of NWA 12241 are similar to the intermediate poikilitic shergottites Lewis Cliff (LEW) 88516 and Allan Hills (ALH) 77005 with increasing ƒO 2 from poikilitic to non-poikilitic tex- tures. Oxygen fugacities and mineral compositions of NWA 12241 indicate similar petrogenesis as the other intermediate and enriched poikilitic shergottites, start- ing with formation of poikilitic textures at crust-mantle boundary depths and formation of non-poikilitic tex- tures during ascent [1–3]. The change from poikilitic to non-poikilitic texture formation is accompanied by degassing and/or autoxidation as evident from the in- crease of melt ƒO 2 . Emplacement of NWA 12241. We conducted quan- titative textural analyses of the olivine population (n = Figure 1: Crystal Size Distribution plot of all poikilitic sher- gottites. Circle = enriched, triangle = intermediate sher- gottites. Other data from [1]. 1762.pdf 51st Lunar and Planetary Science Conference (2020)